you read the title, now listen to my TED talk

thanks to Deathloks, we know that once something owns you soul, it can do some funky things with it.

but if an arch devil owns their soul, *nerd voice* they enter the nine hells of battor has a lemur... *regular voice* thats boring!

your telling me zariel DOSENT transform the souls she owns into deathless soldiers to fight in the blood war?

you're telling me Mammon DOSENT make the souls of those who failed to beat him in a deal into walking bank accounts?

youre telling me levistus DOSENT change the souls he owns into firey monsters to mealt him out of his ice block?

but I only have the DMG for information on the lords of the nine, so if anyone has any other information on lord of the nine I would love to hear it

I would also love to hear any ideas for what an arch devil would make the souls they own into

Sounds like you would enjoy reading Chapter 1 of Mordenkainen's Tome of Foes, where it deals strongly with the Blood War and even has a section on the Lords of the Nine. then later you have Lords and their Thralls with entries on each of the Lords.

Mordekainens tome of foes is a great resource for info about devils and demons. It does talk about how mortals that enter into contracts to sell their soul, then go to the 9 hells when the die. They take the form of some kind of devil based on how powerful they were in life. It says most of these go into legions of devils to fight demons off, so you’re right. Some other important soul to the archdevil might be kept around as a trophy of sorts if that person was sought after by many devils, or had some good or bad relationship to a devil.

The other thing I like to think in my campaigns at that the souls are also forged into powerful weapons or artifacts.  In one villainous campaign I had a character bargain with Fierna that in the exchange for convincing souls to join her armies, that she would bind a powerful recurring enemy into a gun for the gunslinger in my group. There’s a handful examples in campaigns with devils and demons sealed in artifacts, that I think you could toy with too. 
Who knows maybe some devils are different and just like to eat souls in a feast with other devils. Have fun with it!

I can come up with some

• Zariel wants soldiers fast so I imagine she just slaps them in a suit of armor with a weapon and sends them off to fight. Black ichor of melted flesh leaking out of the gaps in the armor as it shambles forward. If you peel them out of the armor, all that is left is a lemure either from being battered and mutated by demons or simply because zariel didn't deem them worth the effort to reconstruct properly.  Use the stat block for animated armor but fiendish 
• Baalzebul is a perfectionist who always starts again and is never finished. I imagine many souls end up mixed up in up together in amalgams to create something beautiful before being discarded or teared apart to start again.  Use creatures like ettin  as a two headed monstrosity or stuff like ghoul\  but obviously made into fiends 
• I think mammon would actually most likely make soul coins from those he souls he captures 
• Mephistopheles uses souls for magical experiments with individuals trapped in horrific curses, left in an eternal flame or half frozen covered in frostbite.  Those that have interesting magical secrets may gain some reprieve by devoulging what they know but as soon as they run out of information to share they return to the torture pits. Use creatures like flameskull or stone cursed 
• Levistus is really into betrayal and schemeing. He likely gives souls as slaves to pit fiends and demons for the sake of loyalty. The majority are probably simply left in a vault or dungeon some where as a bargaining chip or bait for rivals. I think Levistus is probably the most likely to use the traditional lemure 
• Glasya is wild and self indulging.  She also keeps many souls in human form  glamoured up in finery like trophies those she doesn't like or who diappoint her are reduced to the retchid state of a lemure before being discarded. 
• Souls given to Asmodeus are never seen again, no one is sure what he does with them except that it is the worst possible fate.